The present disclosure relates to a motor controller which controls a drive motor, a sheet conveying device including the motor controller, and an image forming apparatus including the sheet conveying device.
Conventionally, an image forming apparatus, such as a copying machine, a printer, and a facsimile which each include an image forming portion, and a multifunction peripheral equipped with respective functions of them, is provided with a plurality of rollers for conveying a sheet on which an image is to be formed. These rollers are driven by a drive motor. As the drive motor which drives the rollers, a servomotor such as a DC brushless motor may be used.
In the case where the servomotor is used as the drive motor, a detector such as a rotary encoder which detects a rotation speed of each roller is normally provided. The motor is supplied with a drive current generated by a PWM method (pulse width modulation method) on the basis of a control signal which commands a rotation condition for the motor which is obtained on the basis of an output signal of the detector. Accordingly, feedback control of a rotation speed of the drive motor is performed.
Incidentally, if a large load is continuously applied to the drive motor, it is possible to perform speed control of the subject to be driven by the drive motor, by increasing a duty of the PWM signal in a range of up to a maximum current (rated load current) that can be supplied to the drive motor. However, when the maximum current is supplied to the drive motor, an amount of heat generated by the drive motor considerably increases, so that the service life of the drive motor may be shortened or the drive motor may be broken.
Meanwhile, if a state where the duty of the PWM signal generated through the feedback control exceeds a predetermined duty continues during a predetermined time, the drive motor may be stopped. In this configuration, if a large load applied to the drive motor is intermittent, the PWM signal having a duty exceeding the predetermined duty is intermittently outputted to the drive motor during the time when the large load is applied to the drive motor. Also in this case, an amount of heat generated by the drive motor can considerably increase. However, in the above configuration in which the drive motor is stopped if the state where the duty of the PWM signal exceeds the predetermined duty continues during the predetermined time, the drive motor is not stopped and continues to operate. Therefore, breakage of the drive motor due to an increase in the amount of heat generated by the drive motor still can occur.